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X-COM: Terror from the Deep is the '95 follow-up to X-COM: UFO Defense (or Enemy Unknown, depending on your nationality) which tries to change as little as possible. After their last failed bid to invade, the aliens on Cydonia sent a the oceans of Earth. Turns out they visited Earth a long time ago, and then never left due to crash-landing. Now, they're upset that humanity had the audacity to deny their space-borne brethren the rite of conquest, so they're picking up where the war left off. They're still UFOs. Just underwater.

TFTD has the look of a "reskin" or an expansion, but is an actual sequel to UFO Defense. It has the same game engine, but the difficulty and map size are increased, making it much lengthier. There are also UI improvements: You can reserve time to kneel as well as shoot, and it remains set for the rest of the battle (or until you un-toggle it), streamlining the tactical phase significantly. Most of these features were bundled into the open source clone of UFO (OpenXCom) in 2010.

UFO cares about you. It wants you to have a good time. There is a bit of cheating involved (reaction time preference), but overall, it treats you nicely if you follow the house rules. Terror from the Deep is another ball of wax. That reaction shot will always hit you, and you won't get a chance of surviving. Chrysalids can fly now. Particle weapons (Laser equivalents) have ammo requirements. Aliens holding one-handed Blaster Launchers. Certain weapons will function underwater but not on land. Multi-part Terror Missions. Only a Lobster Man Commander can get you the game-ending research, rather than an easier Aquatoid or Gillman. Terror from the Deep will put hair on your chest. It is a one-sided alien mosh pit of death and gloom.

An interesting note is that Julian Gollop, one of the two original creators of the first game, had very little to do with this one. Microprose wanted a sequel to the surprise hit UFO Defense, which Julian dutifully started making and would end up becoming XCOM Apocalypse. But it was taking too long for Microprose, so they instead asked an in-house team to work on a sequel using the existing engine from UFO Defense. Gollop's primary contribution is the nature of the ultimate creature in charge of the invasion, but rather than bemoan the fact that Microprose took the game away from him, he incorporated the history of TFTD into Apocalypse, explaining how the end of the game resulted in the founding of Mega-Primus, the city of the setting, but used none of the technology (explaining the lack of sonic weapons).

Firaxis considered remaking Terror from the Deep, but decided to release Enemy Within and XCOM 2 instead. The Stinger to XCOM 2 hinted that a remake of classic TFTD was still a possibility; however, it looks like they decided to explore Apocalypse territory with Chimera Squad instead. Or you can just play the new XCOM with an aquarium in front of your monitor.

For tropes shared with the original X-COM, see X Com UFO Defense.


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  • Achilles' Heel
    • Tentaculats: Unlike the UFO Defense counterpart which can damage tanks, Tentaculats can't damage your SWS, and will still stupidly try to attack them anyway to no avail. So always bring one if you are expecting them.
    • Lobster Men: While even the alien's own Sonic Cannons can barely scratch them, they are surprisingly vulnerable to both melee attack and thermal weapons — if you haven't got the Vibro Blade line of research opened up yet, make sure you pack some Thermal Tazers and/or Thermal Shok Bombs.
    • Triscenes: They can take some Sonic Cannon punishment, but its non-existent underside armor means a single cheap magna-blast grenade thrown under it will most often kill it.
    • Deep Ones: They have a ranged attack, but no reaction fire ability. So it's perfectly safe to position your troops for an optimal shot or run up to them at whack them with melee weapons. They are totally harmless during your turn. Unless you're playing the OpenXcom version, where they can reaction fire.
  • The Aesthetics of Technology: Aliens have a Used Future look.
  • Ambiguous Robots: Tasoths outwardly appear to be lizard men, but autopsy reveals no internal organs and a power cell with some other odd devices where they should be.
  • And I Must Scream: Molecular Control is functionally the same as Psionic Control from the first game. However, the name (and subsequent research) indicates that you are fully aware of what's happening and completely unable to control your body as the aliens make it do whatever they want to do. Aquanauts that recover from Molecular Control have a very high chance of panicking or going berserk.
    • Also, the Bio-Drones. Full stop.
  • Animal Motifs: The smaller USOs have a manta-like design.
  • Animated Armor: Calcinites are giant blobs of protoplasm which seek out antique diving suits strewn across the sea floor. Once they've filled the suit up, it springs backs to "life."
  • Apocalypse How — Continental: Failing to stop the aliens means their city-weapon rises from the deeps and kills pretty much everything. When you win, the aliens still have the last laugh, as the destruction of T'leth results in a near-Planetary apocalypse.
  • Aquatic Mook: With the exception of the few surface-only Terror units, all Aliens are aquatic. All but a few underwater-only Terror units can also function on the surface well enough, so Amphibious Mooks would be more precise.
  • Armless Biped: Like the Reapers, the Triscenes don't have arms. Unlike the Reapers, they make up for it by carrying two Heavy Sonic Cannons strapped to their bodies.
  • Armor-Piercing Attack : While there is an "Armour Piercing" damage type, they aren't good at penetrating armornote . This job instead applies to Vibro Blades, Thermic Lances, and Heavy Thermic Lances. While you can kill the absurdly heavily armored Lobster Men without themnote , once you realize that they take 200% damage from those weapons, you'll be carrying them with you everywhere.
  • Artificial Stupidity: Are any of the aliens in a given mission equipped with Sonic Pulsers? They'll also be carrying either a Vibro Blade, Thermic Lance, or Heavy Thermic Lance — and yet they never use them, even if they've run out of Sonic Pulsersnote . You'd think this would give the player an advantage, but actually it makes the game even more challenging, since instead of trying to make a suicide run against you to get into melee range, they will just spam you with grenades (a typical alien soldier with a melee weapon will usually have at least four grenades on them), which makes them far more deadly than a typical alien armed with a single-shot blaster.
  • Artistic License Physics: Since TFTD is basically just a reskinned mod of the first game, a lot of the issues associated with underwater travel and combat are simply ignored. Ships are capable of traveling at supersonic speeds underwater which, even if you had a ship capable of surviving the feat, would generate massive amounts of turbulance. Your soldiers are able to throw grenades dozens of yards underwater. Furthermore, while powered armor can be used to handwave underwater mobility, your soldiers start the game wearing simple diving suits, yet have no trouble moving around under the crushing weight of hundreds of feet of ocean. Suffering an injury somehow doesn't kill a soldier instantly when their divining suit gets pierced. Also, you can use med-kits to treat injuries and revive unconscious soldiers underwater. Good luck figuring out how that works.
  • Ass Kicks You: Once again, note the curious distribution of defensive power on the Magnetic Ion Armor. The back plate is the second-strongest part of the suit, and the side plates are weakest. While counterintuitive to say the least, this means it is safer to turn your back to the aliens than to let them hit you from the side. Looking at the UFOpedia picture, it's due to that bulky thing hanging off the back.
  • Awesome, but Impractical:
    • Sonic Cannons. Quite a bit more powerful than the Sonic Blasta Rifle (130 vs. 95 respectively) and just a little bit more accurate, they're also much heavier (8 vs. 5), have fewer shots per (10 vs. 15) magazine, it takes up more space (3x2 vs 3x1 squares), and ammunition is much more expensive to create (3 Zrbite vs. 1 Zrbite). You'll want them for the really big enemies, but for the most part, you're better off with the Blasta Rifle.
    • Magnetic Ion Armor. It only offers 10 more armor over the Ion Armor, and while the ability to fly is incredibly helpful, it only works underwater. The benefits do not justify the increased expense (11 more Zrbite than Ion Armor), and you're better off using Ion Armor for most of the game.
    • The Pulse Wave Torpedo. The biggest, strongest sub-mounted weapon, the PWT can shoot down a Dreadnought in two shots. Assuming you hit with both, because those are the only two shots you can have loaded. And those shots cost $28,000 and 4 Zrbite each. While they have an inherently high accuracy, they're really only of use against Dreadnoughts, and those are still manageable by attacking with multiple submarines armed with Sonic Oscillators, which have free ammunition once they're developed.
    • SWS on alien base attacks. Alien bases are full of narrow, one-tile hallways that an SWS cannot pass through. They're helpful on the first part of the mission (especially when it comes to baiting out Tentaculats), but they are completely useless on the second part.
  • Bag of Spilling:
    • X-COM was disbanded after the First Alien War thanks to politics, reduced to an underwater Elerium salvage team financed by a tycoon, before the arrival of aliens prompted the re-constituted Funding Nations to reboot the program.
    • In addition, you will start out with basic weapons and no armor because all the research from the First Alien War is functionally useless underwater: alien alloys react poorly to seawater, meaning they can't be used for armor or weapons (invalidating plasma weaponry), and lasers are impractical underwater due to blooming and other issues (invalidating laser weaponry). Of course none of this explains why you can't simply use the old alien weapons for missions when you aren't underwater, such as terror alerts or base defense, but the game still doesn't let you.
  • Balance Buff: The Manta and Hammerhead subs have a greater range and overall slightly better stats than their equivalents in the original game.
  • Blob Monster: The Calcinites, although they are contained in a humanoid diving suit.
  • Boarding Party: The Cruise Ship and Transport Ship Terror Missions involve X-COM responding to an alien attack on a civilian ship.
  • Boring, but Practical: Sonic Blasta Rifles. The Sonic Cannon is more powerful by a not-insignificant amount, but the weight of the Cannon, as well as the fact that firing it with a snap shot takes 50% of a soldier's Time Units, means that the Cannon is Awesome, but Impractical at best. The Blasta Rifle, on the other hand, has slightly better accuracy, a much more manageable weight, and only uses 33% Time Units when fired with a snap shot. Once you've captured enough of the Blasta Rifles and researched them, they'll be your mainstay weapon for the majority of the game (you'll *still* want Sonic Cannons for the final assault).
  • Brain in a Jar: The Bio-Drones: this is your brain. This is your brain in a jar. This is your brain in a jar after being endlessly tortured. This is your brain in a jar after being endlessly tortured and weaponized so that your screams of terror are capable of melting your fellow humans.
  • Broke Your Arm Punching Out Cthulhu: The end of Terror from the Deep results in T'leth's destruction spreading chemicals all over the world's oceans and thus snapping a few links off the food chain. Oops.
  • The Captain: The highest rank the X-COM soldier can achieve. You can have one Captain for every 30 Aquanauts you employ (and if you're employing enough at the same time to have two Captains, you are massively overstaffed).
  • Commanding Coolness: The second-highest rank the X-COM soldiers can achieve: you can have one Commander for every 23 Aquanauts.
  • The Computer Is a Cheating Bastard: A lot of the same unfair advantages the AI had in the first game as still present in this one, most notably the nightime vision penalty that only applies to your troops and not the aliens, as well as their ability use mind control on your troops even when they are not in visual range. The bug where killing the last alien on the map while some of your troops are under mind control results in them being counted as dead at the end of the level also still applies here. It's also common for missions to start you out too close to the aliens, so they gun you down with reaction fire the moment you step off the transport. Fortunately this one is somewhat more manageable than in the first game, because in this game your transport vehicle starts the level with its door closed, meaning you can safely end the turn inside it before you step out, which causes the surrounding aliens to burn up their time units wandering around, making it much safer for you to disembark from the transport on the second turn.
  • The Computer Is a Lying Bastard: Certain entries in the UFOpedia contain outright lies, such as the illustrations for the Deep One and Triscene carrying Pulse Wave Torpedo Launchers (a fighter weapon, mind you), and the claim that the Hallucinoid can fire freezing bolts (they do not). In the latter case, whether it was a writer's oversight or not is yet unknown.
  • Creepy Old-Fashioned Diving Suit: The Calcinites are old-timey diving suits inhabited by strange, green blobs armed with metal claws. Ironically, they're only found in land-based Terror Missions.
  • Derelict Graveyard: Some tactical maps feature sunken ships or crashed airliners.
  • Diabolus ex Nihilo: The Great Dreamer. Apparently you can't kill it. Nothing can. What works, however, is disrupting its awakening so that it can't wake up and destroy everything; whether this actually kills the Dreamer is unknown. Nobody on Earth can guess where the creature came from as it's a closely-guarded secret: none of the aliens actually know.
  • Diesel Punk: The aliens show elements of this.
  • Divided States of America: The USA is still around, but Alaska has apparently seceded and is now the People's Republic of Alaska.
  • Downer Ending: As highlighted on Pyrrhic Victory, even if the player wins things all go downhill for Earth, culminating in the Crapsack World of Apocalypse.
  • The Dreaded Dreadnought: The Dreadnought is the TFTD equivalent of an alien Battleship.
  • Drone of Dread: The "Geoscape" theme. Indeed, much of the game's soundtrack alternates between this and Scare Chords.
  • Early Game Hell: At the start of the game, money is tight, your single base provides very limited coverage for the world, your technology is massively outclassed by the aliens, your troops lack combat skill, and the lack of armor means a hit from practically anything will kill them. Furthermore, new players may not know what research to prioritize to best bring their forces up to par with the aliens.
    • Special mention goes to the first terror mission in April. It will usually feature Lobster Men and Bio-Drones. The former are almost invulnerable to most early-game weapons, while the latter are armoured miniature flying saucers with a powerful, highly accurate ranged attack.
  • Eldritch Abomination: The Big Bad is the Great Dreamer in T'Leth, and He Waits. You need to go there and make sure He keeps dreaming, since if He wakes up, He'll be unstoppable.
  • Fish People: The Gillmen, being early humans who were genetically modified to survive underwater, and the Lobster Men, being actual lobsters that were genetically modified into bipedal soldiers, both fit. Autopsies reveal that other creatures, like the Deep One and the Hallucinoid, also have some human and aquatic DNA, but they're so far removed from humanity they don't count for this trope.
  • Flying Saucer: Several examples.
    • What do you get when you take the cyberdisc, miniaturize it, waterproof it, replace the CPU with a still-conscious human brain and replace the plasma cannons with a concentrated sound wave generated by said brain's reactions to the inherent agony of the process? The answer is: Bio-Drones. Oh, and they explode when they die.
    • There's also the Dreadnought, TFTD's answer to UFO's Battleship, which almost looks exactly like its UFO counterpart, in that it looks like a giant Flying Saucer.
  • The Future: TFTD is set far enough into the 21st century — that is, far enough to prevent overlap with the previous campaign (regardless of how bad you suck at X-Com, it probably didn't take forty years to complete). The initial alien invasions caused turmoil among the funding nations, splitting some into blocs while annexing others into greater superpowers. Alaska is now a people's republic, China and India formed an Asian Coalition, everything west of the Mississippi has been retaken by Mexico, Europe finally got its act together, and Africa/Egypt are currently governed by private enterprises.
  • Game-Breaking Bug
    • Never research the Tasoth Commander. (Fixed in the CD version.)
    • TFTD has the Research Tree bug, where researching something too early can accidentally block off advancement in that research path. Most crippling is the Live Deep One bug. If you research a Live Deep One before you research the prerequisites for the Ion Armor, you won't be able to research the ships needed to complete the game. Thankfully, most of these have been patched away by the Windows 95 version.
    • If an explosive object is detonated by a stray shot from a Deep One (which is actually extremely unlikely, as the Deep Ones have good accuracy and their shots have a very, very low probability of causing an explosion, usually just melting the object instead), the aliens will freeze up, and their turn never ends.
  • Gameplay and Story Segregation:
    • Molecular Control supposedly works through mind control implants. However, Molecular Control attacks don't require the victim to have a mind control implant: it's implied that this is why MC attacks must be repeated every round (without the implant, there's no lasting control of the victim), but that just raises further questions, such as why your MC-enhanced Aquanauts are not constantly under alien control after getting their own implants.
    • The Interceptor Barracuda not only boasts propulsion that allows supersonic speeds underwater, it's also capable of flight and VTOL. Truly an engineering marvel. (Alien ships also travel ridiculously fast considering they're miles underwater, but at least they have the excuse of using alien technology.
    • Zig-zagged with port attacks:
      1. Your troops, being idiots, are still wearing their weighted shoes and diving suits, losing any dexterity one would expect from fighting on the surface instead of 10,000 ft. below sea level.
      2. Machine guns and heavy artillery aren't available either, for obvious reasons, which is fine because the aliens are restricted to aquatic weapons also. But the Hydrojet Cannon and the Torpedo Launcher, which both use propeller tech to drive their projectiles, are completely ineffective on land.
  • Genre Shift: According to Julian Gollup, after X-Com he was burned out and flat out of juice. He was done with games about aliens, but he expressed interest in developing one based on the Cthulu mythos. Some of these ideas were incorporated into TFTD and Apocalypse.
  • Giant Enemy Lobsters: The much feared Lobster Men. Until you acquire flying suits and mind control. As they are invulnerable to basic pistols, they become the best target practices. Or, when you get Vibro Blades.
  • Glass Cannon
    • Deep Ones possess an powerful gauss-based attack that follows a grenade-like trajectory (it can shoot over walls), but are fairly easy to kill, even with basic weaponry. They do, however, get a lot of attacks.
  • Guide Dang It!:
    • A Deep One corpse is required to get aqua plastic armor, which isn't hard to get, but may not be an obvious leap if you are new to the game and leaves your men naked for longer. Even worse, to research the more advanced armors and watercraft, you need to capture a Deep One alive.
    • Unlike the its predecessor, this game gives you no hints about what aliens are required to get the research topics leading to the endgame. While Enemy Unknown told you to first capture a Leader, and then a Commander, TFTD tells you nothing of the sort, just vaguely implying that the Synomium Devices in Artefact Sites are somehow important to this (they're not). Furthermore, it doesn't tell you that you need to capture aliens of specific rank and race combinations. Therefore, finding the right aliens is a matter of looking up walkthroughs or blind luck you need a Lobster Man Navigator/Commander or Gilman Commander to unlock the first research topic, followed by a Lobster Man Commander for the second topic.
  • Harpoon Gun: Your starting "standard rifle". Five darts at point-blank range won't even faze a Gillman, at least on higher difficulty levels. If you bring a harpoon gun into battle, you can almost hear the aliens laugh. The pistol version, the Dart Gun, is even weaker, and practically useless except for Stat Grinding (more hits to kill an alien = more experience gained).
  • Heavily Armored Mook: Lobster Men are so well armored they can absorb an obscene amount of damage. If you run into these guys before you've researched anything beyond the starting weapons, you should leave immediately. Darts and Harpoons are about as effective against them as tennis balls.
  • Hot Sub-on-Sub Action: Your subs versus USOs.
  • Hover Tank: Displacers, reprising this role from UFO, coming in Sonic Cannon and P.W.T. Launcher flavors. What's more, unlike Magnetic Ion Armor, they can fly on land missions.
  • Humanoid Abomination: The Deep Ones. They're something like an underwater Frankenstein's monster.
  • Human Popsicle: The aliens use cryogenic stasis chambers to remain dormant for thousands of years. You can also sell these chambers as Shop Fodder.
  • An Ice Person: Hallucinoids are prehistoric jellyfish that were modified to use a powerful chemical freezer. They attack by using a melee attack that literally freezes targets to death. The UFOpedia says they can also fire freezing blasts, however they have no such attack.
  • If It Swims, It Flies: USOs (Unidentified Submersible Objects) and your flying subs that intercept them. Both can fly over land, with a handwave stating that the engines are convertible to work in the air without issues, but weapons can only be fired underwater due to their design (a torpedo is not the same thing as a missile, for example).
  • Implacable Man
    • The Lobster Men. They. Will. Not. DIE. Let's put it this way... unless you drop them with the heaviest melee weapons, chances are, they're actually unconscious, not deadnote .
    • The Triscenes are Implacable Dinosaurs, until you find their weakness (their non-existent under armour).
  • Insane Troll Logic: How your troops can survive taking hits that puncture their diving suits, and how they can use med-kits to treat injuries while underwater, is anyone's guess. Though granted, the enemy weapons are _sonic_ guns - it's outright stated in the UFO Paedia that they go straight _through_ the armour (without affecting it), and there's something very special about the alien "plastics" that provides protection to your troops. Even for things like detonation charges, concussion works a lot better under water. Played completely straight for the non-explosive, non-sonic weapons.
  • Ironic Name: Gillman Terror units are named "The Deep One", yet you almost never find them in the ocean (they only appear underwater in Mixed crew Dreadnoughts and during the final mission).
  • Kill It with Ice: Subverted. Freezing weapons, including Thermal Tasers and Thermal-Shok Bombs, are the equivalent of Stun Weapons from Enemy Unknown. Instead of killing targets, they harmlessly freeze them, allowing you to capture aliens.
    • Played straight with the Hallucinoid, however. Despite their attacks sounding like electric shocks, it's stated that their attacks are based on chemical freezers.
  • Lethal Lava Land: Sort of, one of the TFTD combat mission terrains consists of underwater mini-volcanoes leaking cooled lava. They have no effect on your soldiers and enemies, but they do provide illumination in night missions.
  • Lizard Folk: The Psychic Tasoth.
  • Lost Colony: T'Leth is a massive colonizing ship that crashed 65 million years ago.
  • Lovecraft Lite: The alien horde has cousins dwelling under the sea. Their final goal is to awaken the "Ultimate Alien", a giant squid which is entombed in the ancient sunken city of T'leth. And things go as dark and pessimistic as Lovecraft would have written it (only without the scope of a Cosmic Horror Story).
  • Make Me Wanna Shout
    • The Aliens' Sonic Weapons, TFTD's equivalent of UFO's Plasma Weapons.
    • Bonus points for the Bio-Drone, whose sonic beam is based on the original vocal cords of the brain that pilots it, meaning that it literally screams its enemies to death.
  • Master of None: The Gillmen, though they become Jack of All Stats at higher difficulties.
  • Meet the New Boss: Although "not alive but somehow not dead" the Ultimate Alien is subliminally commanding the submarine fleet. It fills the old role left over by the brain.
  • Military Mashup Machine: The alien subs, as well as XCOM subs, are all capable of flying over land. According to the lore, the turbines that drive both spin up and turn into actual jet engines above water, meaning that the subs are actually air/aquatic hybrids. The mounted weapons only work underwater though, which means a sub will pursue a USO until it goes back in the water before attacking.
  • Mooks: The poor Gillmen. They have horrible aiming skills, panic easily, and are not very tough. It is heavily implied they they are enslaved by the other aliens and are considered totally disposable.
  • Mook Commander: As with the last game, you need to capture a commander alive in order to unlock the last level. However, in this game it's much easier, as every base has an unarmed Loberster Man commander on stand-by in the command center. You don't even need to bring any stun weapons at the start of the mission to get him, as there will be plenty of thermal shock launchers available to take from the enemies you encounter on the way to him.
  • Multi-Armed and Dangerous: The Lobster Men, mostly cosmetic in regards of equipment, but not for their close combat ability.

  • Nerf:
    • Dye grenades produce an octopus-like ink spray. Theoretically identical to Smoke Grenades (from UFO Defense), the dye's area of effect is much lower. Instead of providing a compact but effective 6x6 grid of cloud cover, the Dye Grenade creates a tiny puff of dye which spreads across the battlescape over the course of several turns.

      Because cloud cover provides the greatest protection within the first few turns after being released, this pretty much defeats the purpose. A much quicker solution is to simply detonate an explosive for an instantaneous cloud of foam. (It is very likely that the team responsible for TFTD may have mistyped the dye grenade's strength as 10 instead of 100. Some fan patches increase the Grenade's strength)
    • Propeller-driven weapons (specifically the Hydrojet Cannon and the Torpedo Launcher) do not work on the planet surface. The Gas Cannon is therefore your most powerful weapon on land missions, which are the most difficult missions at the beginning of the game, until you have Gauss technology, which will not be available until after your first Terror Site at bestnote .
    • In UFO, Flight Suits would leave Terror Units, such as the fearsome Chryssalid, at your mercy. In Terror From The Deep, Magnetic Ion Armor only allows flight while underwater, and provides a bare 10 points more armor, meaning that regular Ion Armor is the way to go. In addition, the Chryssalid equivalent, the Tentaculat, can fly, which completely invalidates the advantage provided by Magnetic Ion Armor.
    • Gauss weapons. The Gauss Pistol is researchable almost as soon as the game begins, much like the Laser Pistol (UFO), but is less accurate due to the high rate of firenote . Also, unlike the laser pistol, Gauss Weapons chew through ammo at an alarming pace, and your only source of ammunition is what you can manufacture. The Gauss Rifle is much better than the Pistol, being both more accurate and more powerful, but it still has less than half the power of the alien Sonic Blasta-Rifle. The Heavy Gauss is completely useless, providing a minimal power increase with a dramatic weight increase and lowered accuracy, not to mention the loss of Auto Fire. The other, more bitter irony of Gauss technology is that many of the aliens encountered are highly resistant to it, with only Deep Ones, Aquatoids, and Gill Men taking full damage. It's no surprise, then, that most guides recommend ignoring Gauss technology altogether and rushing research on the Sonic Pistol, which is more powerful than the Gauss Riflenote .
    • The Sonic Cannon, when compared to the Heavy Plasma in UFO. While slightly more powerful, it lacks automatic fire capability, fires slowly, and has a small magazine size. This is actually good from a game balance standpoint, since each of the Sonic weapons now has a distinct role. In effect, the Sonic Cannon works like a sniper rifle.
  • New Neo City: "Neo-Japan".
  • No Ontological Inertia: Killing the Big Bad and destroying T'Leth makes all the remaining Zbrite inert, only good enough in large numbers, which is how they managed to send an Avenger to Mars for E-115 prospecting.
  • No Waterproofing in the Future: Weapons and other technology developed (or reverse-engineered) during the decades of fighting in UFO Defense are completely useless underwater, so, in Terror from the Deep, you must restart the researches from scratch. Consider it a Justified Bag of Spilling... that is, unless you're a clever enough hacker to exploit the similarities of the UFO Defense and TFTD engines and carry over goodies whose quantities were stored in the same data addresses. Partially justified by the fact that the alien gear needs Elerium, and the rest of the stuff is lasers.note 
  • Nostalgia Level: Just when you thought it was safe to go to the beach... The aliens will occasionally make landfall and start terrorizing resorts; this is when the game feels most like the original X-Com.
  • Obvious Beta: Terror From the Deep was made by recycling a large part of the coding for the previous game which, although saved some time and money on development, meant that some serious and inherent issues would crop up during a playthrough.
    • It has a lot of crippling bugs. Xcomutil patched most of them, but you get goofy stuff like soldiers disappearing on 2-step missions if they get knocked out, base missions where you can't find the last alien, and the "Tasoth commander bug".
    • Additionally, there are nonsense impediments to some of the important research tree, which lead to more bugs, which aren't game-ending but still make the game needlessly more difficult and time consuming. You need a live Deep One to research Ion Armor (the equivalent of the Power Suit from the original game), so if you roll unlucky and aren't aggressive enough, you may not see ion armor (and subs) until very late in the game since Gillmen quit running terror missions after a couple months.
    • The "Updated Pathfinding AI" in OpenTFTD should make the Cruise Missions etc. a bit less tedious. Warboy discovered that the original algorithm wouldn't let aliens make two 90 degree turns, so that's why the game had so many "closet campers."
    • Converting the unlimited-ammo Laser weapons to limited-ammo Gauss weapons led to a very annoying program oversight with the Gauss Coleacanth.
    The Coelacanth/Gauss does NOT return any remaining ammo to your stores at the end of a mission. In addition, when you assign a Coelacanth/Gauss to a troop transport, 50 Craft Gauss Cannon rounds will be immediately deducted. These rounds cannot be refunded by any means. If you change your mind about assigning it, deassign it, and then reassign it, the premium will be deducted yet again from your stores.
  • Ominous Floating Castle: In the game over sequence, the city of T'leth rises from the sea to work its magic on the human populace.
  • People Jars: Alien Autopsy reports feature the Alien remains in a liquid tank.
  • Phlebotinum Killed the Dinosaurs: The Colony ship T'leth crashing on Earth is what killed the dinosaurs.
  • Power Pincers: The Lobster Men have them, naturally, and they hurt like hell.
  • Punctuation Shaker: T'leth.
  • Pyramid Power: Artifact Sites feature a number of pyramids on the surface section, possibly evoking those from Cydonia. The aliens are looking for these sites to find Synomium Devices to extend their control network.
  • Pyrrhic Victory: Retroactively added in the Apocalypse manual, which states that the Destruction of T'Leth, in addition of killing your elite soldiers, releases the deadly chemicals that instantly kills everything in and around the Gulf of Mexico and turns the rest of the Earth into a toxic wasteland.
  • Rare Random Drop: Averted after the Elerium fiasco of the first game: the destruction of a USO engine does not necessary destroy the associated Zrbite, making it much more common (so much so that, in the early game, you can actually sell it if you're desperately in need of money, since you won't need it for a few months anyway).
  • Reactor Boss: The second part of alien colonies missions, and to lesser extent Artifact Sites, are too large to accomplish by killing every single alien, so it is more practical to find the synonium device that powers the base, destroy it while optionally capturing one of its high ranked guards, and get out.
  • Recycled In Space: Or rather, Underwater
  • Red Sky, Take Warning: Not much has changed since the last game. The Geoscape is still where we spend most of our time, courtesy of a palette swap. The skies are burnt red (perhaps to indicate the aliens are playing hardball now).
  • Reinventing the Wheel:
    • Played With in the subsequent games: because TFTD was not developed by Microprose directly, the technologies from the game are not present in the later games: sonic and gauss weaponry is completely absent, with laser and plasma taking the forefront of XCOM weaponry. Zrbite is mentioned as becoming inert once T'leth was destroyed, possibly providing a Hand Wave, and the inert material becomes important in the development of the hyperdrive in Interceptor.
    • All this doesn't explain why you can't use the old hardware for surface terror missions, especially since the game disabled some of the heavier aquatic weapons, and you can't fly. Lasers would definitely be more useful than harpoons.
  • Reptiles Are Abhorrent: The Tasoth aliens are powerful vaguely reptilian humanoids.
  • Running Gag: A small one amongst veteran Terror players is that bases which see regular combat against the Lobster Men often requisition suspiciously large amounts of butter.
  • Sealed Evil in a Can: The Big Bad is one, not surprising since he is an expy of Cthulhu.
  • Ship Level: Shipping lane attacks and cruise ship terror mission. They consist of two parts, above deck and below deck, like Cydonia. Plus numerous rooms, narrow corridors and lots of hiding places and you got a recipe for disaster.
  • Shout-Out: TFTD is practically made of shout outs, if not direct ripoffs.
    • The Calcinites bear a laughable resemblance to the title antagonist of the B-movie Robot Monster due to them impersonating old-school divers.
    • The Tasoth race are pretty much Lovecraft's Deep Ones (even though there's an entirely different race in the game actually called "Deep Ones"), especially since their original description (which was replaced in the final version of the game) had them being converted humans (much like the aforementioned actual Deep Ones of the final game).
    • The Gill Men are extremely similar visually and thematically to the Gill Man of Creature from the Black Lagoon, as well as being very reminiscent of the Sea Devils and Silurians of Doctor Who.
    • The Great Dreamer, leader of the aliens, who sleeps most of the game away in the sunken spaceship/city called T'leth is, when you finally see him, an expy of Cthulhu, that giant monster/god dude who's slept away most of history in the sunken city called R'lyeh.
    • The Tentaculats are, visually, a copy of D&D Grells, but a lot of that sort of thing went on in the early '90s.
    • The Aquatoids' faces resemble those of the Star-Spawn of Cthulhu, sans facial tentacles.
  • Space-Filling Empire: The Nations in 2040, including:
  • Spell My Name With An S: Is it Gill Man or Gillman? The game doesn't spell their name consistently.
  • Super-Soldier: The Lobster Men, who are extremely resistant to most types of weapon and the Tasoth, who aren't quite as tough, but simply have very high stats (on high difficulty levels, their stats higher than all but the very best human soldiers).
  • This Is a Drill: The game features a series of power drills that are the most effective weapons against Lobster Men. Combine with Molecular Control Disruptors to conserve ammo, or Thermal Shok Launchers/Thermal Tazers if you need to take them alive.
  • Too Dumb to Live: Just like in the last game, civilians in the terror missions just wander around aimlessly and completely ignore the aliens who are gunning them down.
  • Ultra Terrestrials: The Gill Men are a lifeform native to Earth. Research implies that they are a human ancestor that, instead of staying on land, ended up in the water. There's also a lot of evidence of genetic manipulation on the alien's part, so they're a human off-shoot in name only at this point.
  • Under the Sea: Half of TFTD is this.
  • Underwater Base: X-COM's base of operations. Also, one of the rarer terrain type in the USO recovery missions is a small series of underwater modules.
  • Underwater Ruins: Of several varieties. You can find sunken islands, sunken pirate ships (with gold! That you can't have), and even airplane carcasses.
  • Unintentionally Unwinnable:
    • The game was initially released with some brutal bugs in the research tree that could ruin a player's chances of finishing the game without them even realizing it. Thankfully, patches and mods have fixed some of these problems:
    • If a player researches a captured Deep One terrorist before researching a Deep One Corpse, Aqua Plastics, Plastic Aqua Armor, or Ion Beam accelerators, this will make it impossible to acquire Ion armor. Since the most advanced armor is needed to make a submarine that can reach the last level, this renders the game unwinnable.
    • If a player researches a Tasoth Commander before researching the final mission, this will not unlock it and will make it impossible to unlock since other commanders cannot be researched after this.
    • A "sub construction" item must be in your base stores before researching Zrbite and Transmission Resolver. If you research both of these without a sub construction item, there is a chance you won't be able to research sub construction.
  • Used Future: The Aliens seems to give this vibe.
  • The Very Definitely Final Dungeon: The Colony Ship of T'leth.
  • Vibroweapon: The drills.
  • Water Is Air: The Terror from the Deep was directly adapted from the original with no changes, so the characters are able to do ridiculous things like throwing grenades underwater. They also are unable to float or swim (instead just tromping around on the ocean bottom) until you research the equivalent of the flying suitnote . On the Geoscape, there are times where your fighter craft/troop transport cannot engage/deliver soldiers due to the (downed/landed) USO being "too deep", due to water pressure: even the most basic USO is capable of surviving depths that would crush a human-made submarine. Once you have the Leviathan, USO interceptions will never be called off due to extreme depth.
  • Xtreme Kool Letterz: The Sonic-Blasta Rifle and Thermal-Shok Bomb Launcher.
  • Zeerust:
    • CRT monitors in 2040 and much more.
    • Running the X-COM nautical division in TFTD makes you feel like a regular Captain Nemo, particularly with the arsenal of pulp sci-fi weapons. The alien submarines manage to look antique and futuristic in equal measure.